Allow external VoiceChannel client API implementation (#1057)

* ConnectAsync(bool external) API implemented

This allows developers to handle the AudioClient externally (for example with Lavalink)

* Modify ConnectAsync API and add DisconnectAsync to IAudioChannel

* Update summary message on IAudioChannel.DisconnectAsync()

* Review changes:
- Fix `if (!external)` styling
- Remove unecessary ConnectAsync overload

* SocketVoiceChannel overload update

* Update IAudioChannel.ConnectAsync()
- Remove ConfigAction from parameters
- Add SelfDeafen/SelfMute to parameters

* Remove SocketVoiceChannel.ConnectAsync() default params (Inherit)

src/Discord.Net.Rest/Entities/Channels/RestGroupChannel.cs

@@ -147,7 +147,8 @@ namespace Discord.Rest
             => EnterTypingState(options);
 
         //IAudioChannel
-        Task<IAudioClient> IAudioChannel.ConnectAsync(Action<IAudioClient> configAction) { throw new NotSupportedException(); }
+        Task<IAudioClient> IAudioChannel.ConnectAsync(bool selfDeaf, bool selfMute, bool external) { throw new NotSupportedException(); }
+        Task IAudioChannel.DisconnectAsync() { throw new NotSupportedException(); }
 
         //IChannel        
         Task<IUser> IChannel.GetUserAsync(ulong id, CacheMode mode, RequestOptions options)

src/Discord.Net.Rest/Entities/Channels/RestVoiceChannel.cs

@@ -45,7 +45,8 @@ namespace Discord.Rest
         private string DebuggerDisplay => $"{Name} ({Id}, Voice)";
 
         //IAudioChannel
-        Task<IAudioClient> IAudioChannel.ConnectAsync(Action<IAudioClient> configAction) { throw new NotSupportedException(); }
+        Task<IAudioClient> IAudioChannel.ConnectAsync(bool selfDeaf, bool selfMute, bool external) { throw new NotSupportedException(); }
+        Task IAudioChannel.DisconnectAsync() { throw new NotSupportedException(); }
 
         //IGuildChannel
         Task<IGuildUser> IGuildChannel.GetUserAsync(ulong id, CacheMode mode, RequestOptions options)